The way I understood the bridge is that it wasn't a catapult, but a ballista (i.e. catapult-sized crossbow)
Well, if we're going to be pedantic, a crossbow is, first and foremost, a bow. i.e. it has flexible arms, and the energy of the draw is stored in those arms. The Ballista, however, has rigid, spring loaded arms, with the energy stored in the two springs. Thus it is, first and foremost, a catapult, it being essentially two Catapultae lying on their sides, with a string linking the ends of their arms.
For the rest, They were shooting from tree to tree, so I understood that part to work much as it does when passing a line between ships at sea. You shoot a weight on the end of a line (A ballista can shoot rocks as well as spears.) so that the line falls across the deck of the other ship, or in this case, bridging platform. The thing is that if there is too much weight, the shot won't get there, so you have to resort to the well known ploy of attaching a thin line to the weight and then using that to haul a heavier line across. Once you have one line across, you can make a bridge surprisingly quickly, but the description of the bridges being attached directly to the weight and rolling out complete with wooden flooring just seemed a bit Road Runner-ish in comparison to the rest of the story.
Bascule bridges might have worked, and needn't have been much heavier than the catapult system, all told. But on the other hand, perhaps the mental image of the bridges unfurling between trees was just too much fun to pass up.
Edit: Actually, the barb thing would work if you were shooting it into a strong timber put there for that purpose. You could replace the timber when it got splintery.
« Last Edit: July 11, 2015, 03:44:03 AM by SpareInch »
Fresh slush - Shot this morning in the Vale of COW